1. Field of the Invention
The present invention is generally related to the field of position sensors and, more particularly, to rotary position sensors which provide an output signal that is proportional to the angular position of a rotatable member.
2. Description of the Prior Art
In the field of position sensing, many different types of apparatus are well known to those skilled in the art. For example, contact potentiometers are commonly used to provide a very simple method for measuring rotational or rectilinear movement of elements. However, the increased temperature and life requirements in sensor applications, along with the need for much higher resolution, has necessitated the development of new measuring and sensing techniques. In the field of non-contact sensors, the linear variable differential transformer and the rotary variable differential transformer (i.e. LVDT and RVDT) have found acceptance in some applications. Also, the planar coil inductive sensor and the magnetoresistive element are potentially suitable candidates for meeting increasingly more stringent requirements relating to temperature, long life and higher resolution. Each of these techniques also has limitations. For example, RVDT sensors are generally very precise, but they are somewhat limited in their range. In addition, RVDT sensors are occasionally more expensive than a particular sensor application will permit. The planar coil inductive sensors require a relatively sophisticated electronic circuit for operation and are considered by some to be susceptible to electromagnetic interference. The rotary magnetoresistive element sensors are also limited in their range of sensing and must be compensated for temperature variation where wide temperature excursions are expected. Sensors which utilize the Hall Effect have also been used to provide a non-contact means for sensing either rectilinear or rotary motion.
Many different types of non-contact rotary sensors have been developed and are well known to those skilled in the art. For example, U.S. Pat. No. 4,180,753, which issued to Cook on Dec. 25, 1979, describes an apparatus for generating electrically distinguishable bipolar signals using a magnetic sensor and an oval wheel with teeth and notches in its minor and major axes. It makes possible the identification of a subgroup of electrical signals from the remaining electrical signals or from the entire group of signals. The electrical signals can be used together to determine a variable, such as revolutions per minute, representative of a movable element or particular individual signals of the electrically distinguishable signals can be used for initiating a function such as the firing of a spark plug in an automotive application.
U.S. Pat. No. 3,777,273, which issued to Baba et. al. on Dec. 14, 1973, describes an angular position detector which uses magnetic elements. The detector comprises a means for establishing a magnetic field wherein a magnetic flux distributes in such a manner as to vary in accordance with the angular position of a rotary shaft. At least one pair of magnetically affected elements, each having a parameter varied in accordance with a density of a magnetic flux passing therethrough, is placed in the magnetic field and incorporated in an electrical circuit. The electric circuit connects the magnetic effective elements in series to each other and impresses a DC voltage which is then divided into a voltage representing the variation of the parameter of the elements, namely the angular position of the rotary shaft.
U.S. Pat. No. 3,835,373, which issued to Matula on Sep. 10, 1974, discloses a rotational positional sensor which utilizes a Hall device and a means to maintain the Hall voltage constant. The rotational position sensor described in this patent includes a Hall Effect device which is mounted within an air gap of variable size within a magnetic circuit. The Hall Effect device is subjected to changes in magnetic flux density in direct relation to the rotational position of a cylinder member forming a part of the magnetic circuit. The magnetic circuit includes a C-shaped permanent magnet having opposite pole pieces forming air gaps with the cylindrical member. The cylindrical member comprises a half cylinder in the region of the air gap containing the Hall Effect device so as to vary the effective area of the gap and thus the flux density as the cylindrical member turns. The voltage of the Hall Effect device is applied to a feedback circuit including a differential amplifier so as to vary the current through the device as necessary to maintain the Hall voltage constant and provide an output voltage which varies linearly with the rotational position of the cylindrical member.
U.S. Pat. No. 4,570,118, which issued to Tomczak et. al. on Feb. 11, 1986, discloses an angular position transducer that includes permanent magnets and a Hall Effect device. It describes a transducer used for the purpose of creating an electrical signal proportional to the angular position of a member that is pivotally mounted on a given axis. The transducer comprises an element pivoted directly by the member and containing a means for creating a flux field linearly varying in intensity along a given operating line extending in an air gap between spaced portions and having a preselected arcuate shape. A linear Hall Effect device, with an output voltage proportional to the intensity of the flux field to which the device is exposed, is mounted at a fixed position on the operating line and in the air gap. As a result, the output voltage from the Hall Effect device varies proportionally to the position of the Hall Effect device along the operating line as the element is pivoted by the monitored member.
U.S. Pat. No. 4,789,826, which issued to Willett on Dec. 6, 1988, discloses a system for sensing the angular position of a rotatable member through the use of a Hall Effect transducer. The angular position of a member, such as the shaft of a tension arm assembly, is sensed by the combination of a circular type of magnet secured to the rotatable member and selectively polarized relative to its diameter to define a magnetic north-south pole pair and a stationary Hall Effect transducing device secured in close and constant proximity to the ring magnet. A circuit coupled to the transducing device includes offset and amplification stages. In a preferred embodiment, the Hall Effect transducing device is located in the region of a magnetic null of the field generated by the magnetic poles, when the rotatable member is in a selected angular position. The constant gap between the transducing device and the circular magnet optimizes the generation of a linear output through the circuit to accurately sense the member rotation.
In an article by B. D. Inglis and G. W. Donaldson, titled "A New Hall-Effect Synchro", which was published in Solid-State Electronics in 1966, the mathematical analysis of a Hall Effect device is provided. The article also discusses the development of miniature systems in which permanent magnet rotors and Hall plate detectors permit smaller synchros to be made. It describes the possibility that miniature synchros can be produced at acceptable cost while providing a high degree of accuracy.
While Hall Effect devices are well known to those skilled in the art, the application of Hall Effect technology to rotary sensors is usually limited by the type of output signal that is normally provided by the Hall Effect device. For example, although Hall Effect devices are widely used to detect the passage of a magnet past a Hall sensor, such as in an application to count revolutions, the use of Hall Effect technology in an rotary sensor to accurately determine the magnitude of rotation of a rotatable member is severely limited by the nonlinearity of the output signal from the Hall device when the Hall sensor is constructed according to known techniques. In other words, if a generally circular magnet having a north pole and a south pole is rotated about an axis of rotation, a Hall Effect element which is disposed a predefined distance from that axis of rotation will experience a magnetic field whose strength varies as a sinusoidal function of the angular position of the magnet. No portion of the electrical output from the Hall Effect device exhibits any significant degree of linearity with respect to the angular position of the magnet. In some applications, the small portion of the sinusoid that is near the null point of rotation, where the north and south poles have equal effect on the Hall Effect device, can sometimes be used to approximate a straight line. However, these types of approximations must be made with recognition of the fact that errors are incumbent because the output signal is not truly linear. The present invention addresses these problems by providing a rotary position sensor that utilizes a Hall Effect element in cooperation with a shaped magnet to provide a linear output in which the electrical output from the Hall Effect transducer varies proportionally and with a high degree of linearity to the angular position of the magnet.